1. Field of the Invention
The present invention relates to an ion-plating method and an apparatus therefor and, more particularly, to an ion-plating method and an apparatus therefor suitable for forming a film on a wide substrate at high rate.
2. Description of the Related Art
In recent years, attempts have been made to form films by dry processes on wide steel strips such as cold-rolled steel strips to increase the values added. Among these attempts, ion plating is confirmed as a method excellent in adhesion and density of a film and in productivity (Material and Process, Vol. 2, PP. 1636-1637 (1989)). In order to improve productivity in ion plating, a film material must be evaporated at high rate, and a high-power electron gun is advantageously used as a heating means for the material. However, technical difficulties are experienced in ionization of the material evaporated at high rate at a high ionization ratio. No industrial method for a wide steel strip has yet been established.
In formation of a ceramic film by an ion-plating method, a metal material which is a major component of a ceramic is heated, evaporated, and ionized. At the same time, the metal ions are reacted with a reaction gas simultaneously supplied during ionization to form a ceramic film on a substrate. Among ion-plating methods, an HCD (Hollow Cathode Discharge) method is available to simultaneously perform material evaporation and ionization by using a plasma gun. This method is used as a reactive ion-plating method because it has a high ionization ratio. However, since he deposition rate of this method is on the order of one-tenth .mu.m/min, most of the practical systems are small-scale batch systems. An economical disadvantage is inevitably caused when this method is applied to continuous process equipment for a strip.
Published Examined Japanese Patent Application No. 57-57553 proposes a method using an electron gun as a heating mean and arranging a positive electrode near a crucible to improve the ionization ratio and film quality. According to this method, however, since a discharge becomes unstable during film formation at high rate, the method is applied to only small-scale batch systems. It is, therefore, difficult to ion-plate a strip having a large area at high rate by using a wide crucible.
Published Unexamined Japanese Patent Application No. 57-155369 proposes a method of converging vapor particles by a hood located above a crucible and ionizing the vapor particles by a positive electrode and a filament which are located in an upper portion of the hood. According to this method, a stable discharge can be obtained even during high rate evaporation. However, the lifetime of the filament is short, and this method cannot be applied to a practical continuous apparatus.
As shown schematically in FIG. 5, U.S. Pat. No. 4,828,872 (DE 3627 151 Al) proposes a method in which a crucible 3 is entirely covered with a inner chamber 6 having an upper opening 8, and a vapor flow from the opening 8 is ionized by a positive electrode 9 located above the opening 8. According to this method, a stable discharge is obtained even during high rate evaporation, and the arrangement shown in FIG. 5 can withstand a long-term operation. However, since the distance between the electrode 9 above the inner chamber 6 and the crucible 3 is large, thermoelectrons generated by a vapor material 4 cannot be sufficiently accelerated. For this reason, the ionization ratio of the vapor particles is not sufficiently high. In particular, the ionization ratio is largely reduced during high rate evaporation.